1. Field of the Invention
The present invention relates to a single-wavelength semiconductor laser that is applicable to optical communications, optical exchanges, optical recordings, optical operations, optical measurements and the like.
2. Description of Related Background Art
Conventionally, a Fabry-Perot type of semiconductor laser, which uses its cleaved facets as a cavity, has been known. This type of laser has a cavity whose length is several hundred to several thousand times as large as the oscillation wavelength, and has multiple resonance or longitudinal modes in the vicinity of the laser's gain peak. Therefore, the laser is likely to oscillate in a multimode fashion, and such multimode oscillation is particularly prominent during high-speed modulation driving. Multimode oscillation becomes a serious problem when that type of laser is used in fiber optical communications, because of the wavelength dispersion of the optical fiber. Hence, a dynamic single-mode semiconductor laser, which performs single wavelength operation even during high-speed modulation driving, has been earnestly studied and developed.
As a dynamic single-mode semiconductor laser, there have been proposed distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers which introduce a distributed reflector of a grating into the cavity of the laser and utilize its wavelength selectivity in order to selectively reduce the resonator loss of only one resonance mode. However, when a part of the output light beam is reflected and returns to the laser cavity of that dynamic single-mode laser, the laser characteristic will greatly fluctuate even if the amount of the returning light is small. The fluctuation due to the returning light appears as fluctuations of light output power, oscillation mode and oscillation spectral width, with respect to static characteristics. As dynamic characteristics, for example, the intensity noise due to mode-hopping between axial or longitudinal modes of an external resonator increases and the response characteristic during the modulation driving due to the quantum noise varies, because of the returning light.
The operation of the semiconductor laser under the influence of the returning light can be described by using a coupled cavity model. In this model, it is assumed that the multi-reflection at the external resonator is disregarded, and light emitted through the end facet of the laser is reflected by the external reflection plane to be returned to the laser and optically couples to the laser cavity at the emission facet to form the coupled cavity. The operation of the coupled cavity laser can be described by the following generalized van der Pol equation (see Roy Lang and Kohzoh Kobayashi, "External Optical Feedback Effects on Semiconductor Injection Laser Properties", IEEE J. Quantum Electron., vol. QE-16, pp. 347-355, Mar. 1980): EQU dE(t)/dt={i(.omega.(n)-.OMEGA.)+1/2 (G(n)-.GAMMA.)}E(t)+.kappa..sub.ext E(t-.tau.)exp(-i.OMEGA..tau.) (1) EQU dn/dt=-.gamma.n-G(n).vertline.E(t).vertline..sup.2 +P (2)
where .OMEGA. is the oscillation frequency, .omega. is one resonance frequency of the laser cavity, G is the mode gain, .gamma. is the reciprocal of the carrier lifetime due to spontaneous combination, .kappa. is the resonator loss and P is the injection number of carriers per unit volume and unit time, which is proportional to the injected current. In relation (1), the second term represents the contribution of the returning light beam, .tau. is the reciprocal time of light in the external resonator and .kappa..sub.ext is the parameter that indicates the coupling intensity between the resonators and is defined by the following relation: EQU .kappa..sub.ext =(1-R.sub.2).multidot.(R.sub.3 /R.sub.2).sup. 1/2 .multidot.c/2.eta.L.sub.d ( 3)
where R.sub.3 and R.sub.2 are respectively reflection coefficients of the external reflective mirror and the laser facet facing the external reflective mirror, and L.sub.d and .eta. are respectively the laser cavity length and the active region refractive index. The value of .kappa..sub.ext is the amount that can be obtained when light returning into the laser cavity from the end facet of the laser cavity is represented by the combination of light reflected by the end facet and light reflected by the external reflective surface and returning to the laser cavity. In a semiconductor laser, the value of .kappa..sub.ext will increase, and thus the influence of the returning light is likely to appear even when a small external reflection occurs. Specifically, the relative intensity of noise (RIN) increases when the returning light amounts to more than 0.01%. It is known that the increase in RIN is caused by the mode hopping between axial modes of the external resonator (see "Fundamentals of Semiconductor Laser", edited by Japan Academy of Applied Physics, pp. 102-105, published by Ohm Inc., 1987).
Thus, the laser characteristic of a dynamic single mode laser is likely to be subjected to a large fluctuation by the returning light since the returning light is coupled, as the external resonator mode, to the oscillation mode of the laser. As a result, stabilization of light output and suppression of the intensity noise are insufficient if effected only by the establishment of a single mode in the laser cavity. Therefore, when the laser is used as a light source in fiber optical communication systems, the use of an optical isolator is indispensable.
As discussed in the foregoing, a conventional single mode laser is unstable for returning light, and hence the isolator needs to be used. Therefore, coupling loss due to the insertion of the isolator occurs, the size of the device increases, and the cost of an entire apparatus rises.